导航栏触碰之后闪烁两下就没了

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

<meta http-equiv="Content-Type" content="text/html; charset=GBK" />

<title>无标题文档</title>

<link rel="stylesheet" type="text/css" href="index.css" />

<script type="text/javascript" src="jquery-1.4.2.js"></script>

<script type="text/javascript" src="slide.js"></script>

</head>

<body style="content:inherit;">

<div id="header">



 

<ul id="nav">

    <li class="mainlevel"><span class="note">主页</span></li> 

    <li class="mainlevel" id="mainlevel_01"><a href="#">企业文化</a>

    <ul class="sub_nav_01">

    <li><a href="#">企业文化</a></li>

    <li><a href="#">主页</a></li>

    <li><a href="#">主页</a></li>

    <li><a href="#">主页</a></li>

    <li><a href="#">主页</a></li>

    </ul>

    </li>

    <li class="mainlevel" id="mainlevel_02"><a href="#">专家内容</a>

    <ul class="sub_nav_01">

    <li><a href="#">主页</a></li>

    <li><a href="#">专家内容</a></li>

    <li><a href="#">专家内</a></li>

    <li><a href="#">主页</a></li>

    <li><a href="#">主页</a></li>

    </ul>

    </li> 

    <li class="mainlevel"><a href="#">联系我们</a>

    <ul class="sub_nav_01">

    <li><a href="#">联系我们</a></li>

    <li><a href="#">联系</a></li>

    <li><a href="#">联系-FO</a></li>

    <li><a href="#">联系</a></li>

    <li><a href="#">联系</a></li>

    </ul>

    </li>

    <li class="mainlevel"><a href="#">企业介绍</a><!--input an em tag as a space,IE is gread need-->

    <ul class="sub_nav_01">

     

    <li><a href="#">联系</a></li>

    <li><a href="#">企业介</a></li>

    <li><a href="#">企业介绍</a></li>

    <li><a href="#">企业介绍</a></li>

    <li><a href="#">企业介绍</a></li>

    </ul>

    </li>

    <li class="mainlevel"><a href="#">产品介绍</a>

    <ul class="sub_nav_01">

    <li><a href="#">产品介绍</a></li>

    <li><a href="#">产品介绍</a></li>

    <li><a href="#">产品介绍</a></li>

    <li><a href="#">产品介绍</a></li>

    </ul>

    </li>

</ul>

</div>

</div>



<div id="head">

</div>



<div id="main">



<div id="float"></div>

<div id="cent"></div>

<div id="reg"></div>



</div>



<div id="footer">

</div>

</body>

</html>







/* CSS Document */

body{ width:1100px; font-size:14px;}



#header{ width:1100px; height:80px;  border:solid 1px #FF0000; background-color:#d3dfe0; }

#main{ width:1100px; height:275px;border:solid 1px #FF0000;} 

#head{  width:1100px;height:350px;border:solid 1px #FF0000;}

#float{ width:265px; height:175px;border:solid 1px #FF0000; margin:45px; float:right; }

#cent{ width:415px; height:180px;border:solid 1px #FF0000;margin:45px; float:right;}

#reg{ width:135px; height:175px;border:solid 1px #FF0000;margin:45px; float:right;}



#footer{ width:1100px; height:150px;border:solid 1px #FF0000;}



 

#nav .mainlevel {float:left; text-align:center; display:block;font-size:16px;}

#nav .mainlevel a { color:#707071; text-decoration:none; line-height:30px; height:34px; text-align:center; padding:0 20px; display:block; _width:48px;font-weight: bold;}

#nav .mainlevel a:hover {text-decoration:none;color:#131313; background:url(Bt008_007_01_0006s_0000_ON.png) 15px 30px repeat-x;}

#nav .mainlevel ul {position:absolute; display:none; *width:2000px;/*IE is great need, width>=li.length*/margin: 6px -130px;border:solid 1px #FF0000; height:25px; }

#nav .mainlevel li {float:left;height:50px; background:url(Bt008_007_01_0000s_0004_DOT-LINE.png) no-repeat 1px 10px;}

#nav .mainlevel li a {padding:0 12px; line-height:30px; height:24px; display:block; _padding-bottom:6px;/*IE6 only*/font-size:14px;} 

#nav .mainlevel li a:hover { text-decoration:none;}

.note { color:#707071;display:block; line-height:34px; padding:0 3em;}

#nav { margin-right: 200px;margin-top: 40px;display:block;}

ul,li {list-style-type:none; float:right; }

//menu

$(document).ready(function(){

  

  $('li.mainlevel').mousemove(function(){

  $(this).find('ul').slideDown();//you can give it a speed

  });

  $('li.mainlevel').mouseleave(function(){

  $(this).find('ul').slideUp("fast"); 

  });

  

});

我写的好好的，测试了一下然后就闪烁了两下不停留了,
我没加样式之前好好的，加了点图片什么的就变成触碰闪烁两下，
麻烦大神看看哪里错了,为毛好好的突然二级导航触碰不停留了.
我用的是jQuery1.4.2 jQuery 官网下的